package org.lisy.designpattern.interpreter;

import java.util.HashMap;
import java.util.Stack;

/**
 * 解析输入公式
 * 
 * @author lisy
 */
public class Calculator {

	private Expression expression;

	public Calculator(String expStr) {
		// 定义一个堆栈，安排运算的先后顺序
		Stack<Expression> stack = new Stack<Expression>();
		// 表达式拆分为字符数组
		char[] charArray = expStr.toCharArray();
		// 运算
		Expression left = null;
		Expression right = null;
		for (int i = 0; i < charArray.length; i++) {
			switch (charArray[i]) {
			case '+': // 加法
				left = stack.pop();
				right = new VarExpression(String.valueOf(charArray[++i]));
				stack.push(new AddExpression(left, right));
				break;

			case '-': // 减法
				left = stack.pop();
				right = new VarExpression(String.valueOf(charArray[++i]));
				stack.push(new SubExpression(left, right));
				break;
			default: // 公式中的变量
				stack.push(new VarExpression(String.valueOf(charArray[i])));
			}
		}
		// 把运算结果抛出来
		this.expression = stack.pop();
	}

	// 计算结果
	public int calculate(HashMap<String, Integer> var) {
		return this.expression.interprete(var);
	}
}
